1. Field of the Invention
The present invention relates to a contact charging system in an image forming apparatus and in particular, relates to stabilization of a charging performance.
2. Description of the Related Art
In recent years, injection charging which is a charging system not accompanied with discharging is watched as a charging system on an image carrier surface in an image forming apparatus.
The injection charging is excellent in charging efficiency. For example, in non-contact charging, in order to charge a surface of a body to be charged at −500 V, it is required to apply a bias of approximately −800 to 1,200 V to a charging unit, whereas in the injection charging, only a bias of approximately −500 to −700 V is necessary and it does not follow the Paschen's law of discharging, and therefore, the generation of ozone due to discharging is remarkably low.
Also, in recent years, a method of using a charging auxiliary particle is proposed as a method of stably performing injection charging. This stabilizes charging characteristics by mediating a charging auxiliary particle between an elastic roller or a brush roller and a photoconductor, and in general, the injection charging efficiency is improved by using a particle having a smaller particle size than a toner and having low resistivity.
For example, JP-A-2005-326659 discloses an example for improving the efficiency of injection charging by combining a charging member having a specified expanded cell and a conductive charging auxiliary particle. Here, the charging auxiliary particle is externally added to a toner in a development unit in advance, and since the charging auxiliary particle is a conductive particle, it remains on the photoconductor without being transferred in a transfer step. In addition, the disclosed example is concerned with a cleaner-less process not having a photoconductor cleaner, and after it has functioned as a charging auxiliary particle in a charging section, the particle is recovered in a development section.
Also, JP-A-2005-99550 discloses an example for applying a charging auxiliary particle to a brush charging roller.
This example is concerned with a system in which a charging auxiliary particle is contained in a brush in advance and the charging auxiliary particle is not externally added in a toner in advance. Furthermore, this example is concerned with a configuration provided with a photoconductor cleaner, the resistivity of the charging auxiliary particle is also set up on a lower level than the toner, and the injection charging is carried out by the charging auxiliary particle.
As described above, though some examples of using a charging auxiliary particle have been proposed, there are still involved the following problems.
(1) In all of these examples, a charging performance of injection charging itself is insufficient, and when a halftone image or the like is printed, there may be a possibility that streak unevenness is generated due to charging unevenness.
(2) In the system of externally adding a conductive particle in a toner in advance, the charging characteristics of the toner are deteriorated by the charging auxiliary particle, and an image with high image quality cannot be obtained as compared with the case where the charging auxiliary particle is not present.(3) In the system of not mixing a charging auxiliary particle in a toner, since even a trace amount of the charging auxiliary particle is always released from a charging unit, the charging auxiliary particle intermixes into a development unit, thereby deteriorating the charging characteristics of the toner. Thus, when used over a long period of time, the image quality is deteriorated.